This invention relates to wet process manufactured phosphoric acid and more particularly to a method for purifying wet process manufactured phosphoric acid.
The bulk of phosphoric acid is produced by the so-called wet process method. Although there are a wide variety of wet process methods, they all basically comprise the acidulation of phosphate bearing rock with sulfuric acid which results in the precipitation of calcium sulfate and the release of crude phosphoric acid. The crude acid thus produced from the so-called "gypsum" method contains from 22 to 34 percent (as P.sub.2 O.sub.5) phosphate (the "hemihydrate" method produces about 30% to about 45% P.sub.2 O.sub.5 acid) and is subsequently concentrated by evaporation to a phosphate content (expressed as P.sub.2 O.sub.5) of about 50% (e.g., "merchant grade"). The concentrated crude phosphoric acid is subsequently utilized in the production of fertilizers or is subsequently treated to produce high grade phosphoric acid or other phosphates.
Another process called the "furnace process" involves reduction of beneficiated phosphate rock in an electric furnace to produce phosphorous, which is then oxidized to P.sub.2 O.sub.5. The P.sub.2 O.sub.5 is dissolved in water to make aqueous phosphoric acid. Another process involving reduction and oxidation, in a rotary kiln, is that of U.S. Pat. No. 3,241,917 to Lappel.
The crude phosphoric acid is relatively high in impurities including aluminum, magnesium and iron complexes. These impurities, which are suspended in the crude acid solution, are not normally removed during the acid manufacturing process, nor are they removed by holding the crude acid in a settling tank for a commercial reasonable time, i.e., on the order of 48 to 60 hours. Consequently, the impurities are encountered in the form of a sludge in the bottom of a tank car after shipping the acid from the manufacturing site to the customer's location. The presence of this sludge often results in the rejection of the shipment by the customer or in the loss of a substantial portion of the crude acid when decantation of the clear acid is attempted.
In addition to the above impurities, crude acid contains substantial proportions of fluorine. The presence of fluorine is considered undesirable, particularly if the phosphoric acid is to be used for purposes other than in the production of fertilizer.
Certain present methods for purifying the crude phosphoric acid, such as, for example, the addition of potassium ferro cyanide to remove iron and vanadium or the use of sodium silicate to remove fluorine, add substantially to the expense of manufacturing the crude acid.